The art of mechanical structures has seen the development of a wide variety of different approaches to try to gain high strength with low weight. Early work was directed toward trying to improve structures made of steel or other high strength metallic components. Later work included the incorporation of low weight metals or the like. Even cellular plastic with reinforced densified plastic skin was tried to gain high strength and low weight, such as for use in aircraft structures or the like. One of the most recent and most successful breakthroughs occurred in the use of laminated composites, also sometimes referred to as composite laminates. In this approach, a lightweight high strength fiber, such as carbon or the like, is included in a resinous matrix to form individual unidirectional layers with all fibers parallel in a given layer and a plurality of the unidirectional layers are put together in order to get the high tensile strength of the filament along various orientational angles. This type of structure has been particularly successful in supersonic aircraft structures, such as wings and the like. One problem, however, has been the delamination in which a delamination fault occurring between layers or within a given layer of the laminated composite material has severely reduced the compressive strength of the composite laminate.
Since the composite laminates are relatively new, the defects, such as the delaminations, and correcting them has not been described in many publications yet. These delaminations often occur during the service life of a structure made from such composite materials and can result in significant loss of strength, as indicated. By arresting the delaminations and thus limiting them to a benign size, this invention allows the structure to perform at the desired level of strength throughout its service life.
The prior art has seen two early methods attempted to correct the delamination. The first method was the placement of stitches of thread-like material through the composite laminate in the thickness direction during the manufacturing process to provide resistance to delamination between layers. The second method was the placement of a continuous film of adhesive between every two layers of composite material to provide resistance to delamination by bonding each layer to its nearest neighbors.
The first method, stitching the layers together, has a disadvantage of reducing the structural properties of composite laminate as a result of the holes created by the stitching process. Both the tensile strength and the compressive strength in the plane of the laminate are seriously degraded as a result of this process.
The second method, placing continuous adhesive film between the layers, is effective for improving delamination resistance for one mode of propagation (the relative motion between the layers in the sliding sense- called Mode II in fracture mechanics terminology), but offers no improvement in delamination resistance or the other mode of propagation (peeling apart of the layers from each other- called Mode I in fracture mechanics terminology).
It is desirable that the method of arresting delamination be effective in both Mode I and Mode II increasing the resistance of delamination without causing significant degradation in the in-plane strength property of a composite laminate.
Yet, the prior art has now provided a method of effecting such desired end result.